[Salvaged from the old site - cut/paste papaw : )]


Note: In the United States prior to 1929 the sizes stamped on wrenches usually referred to the diameter of the bolt not the actual opening size. Thus a wrench stamped ½ U. S. would actually have a 7/8" wrench opening size as a nut for a ½" diameter U. S. Standard bolt would measure 13/16" across the flats and allowing for 1/16 clearance would require a 7/8" wrench opening. This same size wrench would also fit nuts for 5/8" hex cap screw and bolt and nuts for 9/16" S.A.E Standard Cap Screws and thus would be marked ½ U.S, 5/8 Hex Cap, and 9/16 SAE.

Carl Bolt's Antique Tool Museum has an exhibit with 17 wrenches with 19 different identifications and sizes all marked 1/2"! This could get very confusing so circa 1929 the American Engineering Standards Committee specified that wrenches should be marked with the actual width across the flats of the nut or bolt head they would fit

AF - (Across Flats) Also marked with size in inches of nut or bolt head i.e. I/2 AF. (British?). Size as measured across the flat sides of nut or bolt head or actual wrench opening size.

Hex logo and initial C - (Hex Cap Screw). Also marked with size in inches of bolt diameter i.e. I/2 OC. Found on older forged wrenches before 1920 standard want into effect (see note above). A wrench marked 1/2 OC should have a jaw opening of 3/4".

SAE - (Society of Automotive Engineers). Also marked with size in inches of bolt diameter i.e. I/2 SAE. Found on older forged wrenches before the 1929 standard want into effect (see note above). Society of Automotive Engineers Standards were for fine thread nuts and bolts. A wrench marked 1/2" SAE should have a 3/4" jaw opening.

USS - (United States Standard) Also marked with size in inches of bolt diameter i.e. I/2 USS) Found on older forged wrenches before the 1929 standard want into effect (see note above). A wrench marked 1/2 USS would have a 7/8" jaw opening.

From Don "Bus" Haury's Guide for Identifying Markings Found on Collectible Wrenches on the The Wrenching News Website
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The same in the UK - but here we had BSW (British Standard Whitworth) and BSF (British Standard Fine) all with 55 degree thread angle- plus the BA (British Association) an oddity with metric pitches and 47.5 degrees thread angle - spanner sizes were marked thus: 1/2 BSF 9/16 BSF - the finer BSF thread uses a size smaller spanner than the same size BSW to avoid over torqueing. About the 1960's we started to use the UNC and UNF (SAE) threads and A/F spanners, and now we use mainly SI (system International) metric threads with 60 degree thread angle, used with metric A/F spanners... My current tool kit is metric, I drive an Italian Fiat - but still have my old spanners for my vintage 1950's tractors...

This was very helpful information that I did not have.   Rusty - appreciate you digging it out from the dead.  For a newbie (such as myself) it may save the experienced one (YOU) a question or three.